We have purified seven cytochrome P-450 isozymes (UT-2, UT-3, Ut-4, UT-5, UT-7, UT-11 and UT-12) from male untreated rats and five cytochrome P-450 isozymes (Dex I, Dex II, Dex III, Dex IV, and female Dex II) from male and female rats, treated with dexamethazone. UT-5, however, appears to be identical to Dex IV. These eleven isozymes are different isozymes judging from many characteristics. We have previously reported another five different isozymes purified from 3-methylcholanthrene and phenobarbital-treated rat liver. In all we have purified sixteen different isozymes from rat liver. Four of the eleven isozymes represent a new group of isozymes (UT-12, Dex I, Dex II and Dex III). One isozyme (Dex III) possesses catalytic activity for testosterone 6 beta-hydroxylation. Purified Dex III, however, is easily denatured, but anti-rabbit antibody raised against it inhibited testosterone 6 beta-hydroxylation. 2) We determined the coding nucleotide sequence of the mRNA for cytochrome P-450 UT-2 of rat liver by sequence analysis of cloned cDNAs. The amino acid composition of the deduced sequence also agrees well with that determined from the purified protein. Computer-aided analysis was carried out to compare the complete primary structure of another species of cytochrome P-450. The influence of age, sex and inducers on the expression of the isozymes was evaluated by hybridization of mRNA, catalytic activity and immunochemical reactions.